Optical recording media include rewritable optical discs such as a magneto-optical disk and a phase-change optical disk. Typical recording techniques for recording the record data on such an optical recording medium include a mark position recording technique wherein the position of the record mark has information, and a mark edge recording technique wherein each of the front edge and rear edge of the record mark has information.
The mark edge recording technique is suited to high recording density; however, the length of the record mark must be controlled with a higher accuracy so as to allow the record data to be reproduced with a higher fidelity. The length of the record mark on the magneto-optical recording disk or the phase-change optical disk is determined by a temperature rise of a recording film caused by irradiation with laser light. The temperature rise during the irradiation of the optical disk with the laser light is changed depending on the structure and the linear speed of the disk.
During recording the record data on the optical recording disk, a recording technique, or so-called recording strategy technique, is generally used, wherein the waveform of the record data is divided into a plurality of short pulses. For controlling the temperature rise to control the mark length with a higher accuracy, the intensity and the width of each of the plurality of short pulses in the divided recording pulse waveform (laser-modulated pulse waveform) must be optimized depending on the structure of the disk. “Shingaku Technical Report MR93-55, CPM93-107, pp 13-18, 1993” describes an example of the techniques for optimizing the recording strategy such as the pulse width or pulse interval. In this technique, the lengths (time intervals) between the reference mark and the front and rear edges of the subject mark to be measured are measured to determine the irradiation starting point and the pulse width of the recording pulse waveform.
However, as the record mark length recorded on the disk is reduced along with the development of the higher recording density, there sometimes occurs a case where the signal amplitude of the reproduced waveform is reduced down to a level below the slice level of a slicer used for measuring the record mark. This results in that the positions of the front edge and rear edge of the subject mark cannot be measured with a higher accuracy, and thus it is difficult to optimize the recording strategy by using the conventional technique such as described above.
As a technique for optimizing the recording strategy under the condition of a high recording density, JP-A-2001-126260 describes a technique for deriving a pulse response from the reproduced waveform in the premise that the recording/reproducing system is linear, to optimize the recording strategy. According to this conventional technique, hj providing minimum values for ε′ are obtained as time-series data of the pulse response, the ε′ being expressed by the following formula (1):
                                          ɛ            ′                    =                                    ∑              k                        ⁢                                          (                                                      y                    l                                    -                                                            ∑                      j                                        ⁢                                                                  a                                                  l                          -                          j                                                                    ×                                              h                        j                                                                                            )                            2                                      ,                            (        1        )            
wherein ai is a record data such as expressed by “1” or “0”, yi are time-series data obtained by sampling the reproduced waveform based on the clock frequency of the record data. The range for “j” is determined by the range where the time-series data hj assume non-zero finite values, and “k” is determined by the number of all the time-series data of the reproduced waveform. Subsequently, hj and the minimum values of ε′ in each recording pulse waveform are similarly derived by changing the each recording pulse waveform, and the recording pulse waveform providing the smallest value among the minimum values of ε′ is determined as the optimum recording pulse waveform.
In a linear recording/reproducing system, assuming that hj is an output (generally referred to as “pulse response” or “impulse response”) of the recording/reproducing system during recording/reproducing a 1-bit data, the reproduced waveform output at a specified time is expressed, if there is no noise, by the following formula (2):Σ(ai-j×hj)   (2).The pulse response assumes different values for the recording densities and beam diameters or recording/reproducing conditions (such as tilt and defocus). The above ε′ is the index for evaluating the nonlinear components of the reproduced waveform, wherein a smaller value for ε′ means a higher linearity of the reproduced waveform.
However, in the conventional technique as described above, if the recording density is extremely high to extensively reduce the signal amplitude of the reproduced waveform, the influence by noise cannot be neglected, and accordingly, the accuracy of the time-series data hj of the pulse response and the minimum values of ε′ is degraded, whereby there arise the problem that the optimization of the recording strategy is difficult.
In the conventional technique as described above, the clock frequency is extracted from the reproduced waveform, and the sampling of the reproduced waveform is performed using this clock frequency. The extraction of the clock frequency from the reproduced waveform needs a PLL circuit, to thereby complicate the circuit structure. In addition, data having a data length of around 1000 bits, for example, is needed depending on the circuit performance of the PLL, whereby there arises the problem that the signal processing takes a long time.
Although optimization of the recording pulse width is mainly described in the conventional technique, the PLL circuit does not necessarily operate in order for adjusting the laser power if the laser power is inadequate, to raise the problem that the pulse response cannot be derived. Moreover, although the conventional technique uses only the absolute value (the above ε′) of the deviation from the linearity, there arises a need for normalizing the deviation from the linearity in some format, because the signal amplitude changes together with the change of the recording power in consideration of the adjustment of the recording power.